The purpose is to determine if angiotensin converting-enzyme (ACE) inhibition affects arterial and cardiopulmonary baroreflex control of the heart and circulation. The acute effect of I.V. enalapril on baroreflex function is being studied. Arterial baroreflex function is assessed by the heart rate and sympathetic nerve activity responses to changes in arterial pressure. Cardiopulmonary baroreflex function is assessed by responses to increases/decreases of central venous pressure.